This is due to changing requirements to the Product Advertising API. The AssociateTag parameter is now a required and validated field. You can see in the list of changes that it is the first thing mentioned.

These changes are made to keep the Product Advertising API in line with its purpose of driving affiliate sales through the Amazon.com website. The AssociateTag is a tag generated from your Amazon Associates account.

So far, it looks like the tag is not entirely validated to your account. I’ve had luck using fictitious tags such as aztag-20.

Thanks for this and I can confirm the dummy associates tag will work! I’ve got my code working OK now thanks but you have to use your own ID otherwise you won’t get any money will you, or will Amazon be able to figure out your account details via your public and private keys?